Liquid-filled transformers have historically used cellulose paper as a primary solid electrical sheet insulation. Cellulose paper has several shortcomings, such as moisture absorption, water generation, and limited thermal capabilities. Cellulose paper must be thoroughly dried prior to impregnation under vacuum with a transformer or dielectric liquid. Accordingly, the manufacturing process for high voltage transformers with liquid impregnated cellulose paper is lengthy and labor intensive. Following the heat and vacuum process, the cellulose is typically impregnated with mineral oil to slow the re-absorption of moisture. Water generation occurs as the cellulose ages due to heat. Water generation results in reduced dielectric strength of the oil, and may eventually cause a transformer to fail.
High voltage transformers must be manufactured to very precise dimensional tolerances. Dimensional instability can produce significant electrical losses. Cellulose materials also exhibit a high degree of mechanical creep and measurable deformation from long term static loads and dynamic loads. Additionally, natural cellulose can react with transformer oils to form acid by-products which in turn can cause accelerated degradation of electrical insulation.
In view of these shortcomings of cellulose paper, there is a need in the field for improved barrier materials for use in high voltage liquid-filled transformers.